In an electronic circuit such as an IC or an LSI which is mounted on a substrate, high integration is advanced to realize high speed and high function, and accordingly, a large electric current is consumed. According to heat generation of the large electric current, a heat sink is always provided on the electronic circuit to prevent the electronic circuit from exceeding an allowable temperature. For example, the heat sink is configured of a metal, effectively radiates the heat generated by the electronic circuit to the atmosphere, and thus, suppresses an increase in the temperature of the electronic circuit.
When the heat sink is provided, high frequency components of electromagnetic waves generated by the electronic circuit combine with the heat sink, and when the heat sink is a resonant state, there is a problem that significantly strong electromagnetic radiation noise (hereinafter, referred to as “radiation noise”) is radiated to the atmosphere. Since the radiation noise decreases radio performance of a device, a unit which suppresses the radiation noise is required.
In order to solve the problem of the radiation noise, there is a method which inserts an electromagnetic wave absorbing material between the electronic circuit and the heat sink. However, in order to obtain sufficient electromagnetic wave absorbing effects by the electromagnetic wave absorbing material of the related art, the electromagnetic wave absorbing material should have a thickness in a certain extent. Accordingly, a decrease in a thermal contact between the electronic circuit and the heat sink is generated, and thus, there is a problem that the electronic circuit cannot be sufficiently cooled.
In Patent Document 1, an electromagnetic wave absorbing material which alleviates the influences of the above-described problems is described. As shown in FIG. 24, an electromagnetic wave absorbing material 1 is formed of a sheet shape, and has a structure in which a first layer 11 formed of a composite magnetic material having high relative permeability and a second layer 12 formed of a composite dielectric material having high relative permittivity are laminated to each other.
In Patent Document 1, it is described that electromagnetic wave countermeasure effects are sufficiently obtained by the electromagnetic wave absorbing material 1 even though the sheet is thinned. Moreover, it is described that a decrease in the thermal contact can be suppressed since the sheet can be thinned. Furthermore, it is described that adjustment of absorption frequency of the electromagnetic wave is performed by a relative permeability control according to adjustment of a compounding ratio of magnetic materials configuring the composite magnetic material and a relative permittivity control according to adjustment of a compounding ratio of dielectric materials configuring the composite dielectric material.
In Patent Document 2, another method for solving the problems of the radiation noise is described. That is, as shown in FIG. 25, a heat sink device is obtained in which an inner layer pattern 121, a fitting pattern 13, amounting stud 14, and a heat sink 3 can be the same potential by connecting the inner layer pattern 121 and the heat sink 3 having a common potential to each other, and radiation noise can be attenuated by electrostatic shield (shielding) effects. Furthermore, the heat sink device is obtained which realizes space saving by making a pad diameter of the fitting pattern 13 and a diameter of the mounting stud 14 be approximately the same as each other and is suitable for a high density mounting.